This invention relates to a valve timing control apparatus for use in an internal combustion engine to provide a desired valve timing(s) according to engine operating conditions.
For example, U.S. Pat. No. 4,231,330 discloses a valve timing control apparatus for controlling the intake or exhaust valve timing according to engine operating conditions. The valve timing control apparatus employs a cylindrical gear having internal and external threaded portions one of which is taken in the form of a helical gear. The external threaded portion engages with the inner threaded portion of a rotary drum which is formed on its outer peripheral surface with sprocket teeth for engagement with a timing chain engaging with a sprocket mounted on the crankshaft of the engine. The internal threaded portion of the cylindrical gear engages with the external threaded portion formed on the camshaft of the engine. A return spring is provided to urge the cylindrical gear to rotate the camshaft with respect to the rotary drum in a direction retarding the valve timing. The valve timing is advanced by introducing a fluid pressure into a pressure chamber formed on one side of the cylindrical gear to move the cylindrical gear against the resilient force of the return spring.
With such a conventional valve timing control apparatus, however, the valve timing control is made merely in an ON-OFF manner between two valve timing values.